The present invention relates to steel cables (xe2x80x9csteel cordsxe2x80x9d) which can be used for reinforcing rubber articles such as tires. It relates more particularly to the cables referred to as xe2x80x9clayeredxe2x80x9d cables which can be used for reinforcing the carcass reinforcements of tires for industrial vehicles such as xe2x80x9ctruckxe2x80x9d tires.
Steel cables for tires, as a general rule, are formed of wires of perlitic (or ferro-perlitic) carbon steel, hereinafter referred to as xe2x80x9ccarbon steelxe2x80x9d, the carbon content of which is generally between 0.2% and 1.2%, the diameter of these wires most frequently being between 0.10 and 0.40 mm (millimeters). A very high tensile strength is required of these wires, generally greater than 2000 MPa, preferably greater than 2500 MPa, which is obtained owing to the structural hardening which occurs during the phase of work-hardening of the wires. These wires are then assembled in the form of cables or strands, which requires the steels used also to have sufficient ductility in torsion to withstand the various cabling operations.
For reinforcing carcass reinforcements of truck tires, nowadays most frequently so-called xe2x80x9clayeredxe2x80x9d steel cables (xe2x80x9clayered cordsxe2x80x9d) or xe2x80x9cmulti-layerxe2x80x9d steel cables formed of a central core and one or more concentric layers of wires arranged around this core. These layered cables, which favor greater contact lengths between the wires, are preferred to the older xe2x80x9cstrandedxe2x80x9d cables (xe2x80x9cstrand cordsxe2x80x9d) owing firstly to greater compactness, and secondly to lesser sensitivity to wear by fretting. Among layered cables, a distinction is made in particular, in known manner, between compact-structured cables and cables having tubular or cylindrical layers.
The layered cables most widely found in the carcasses of truck tires are cables of the formula (L+M) or (L+M+N), the latter generally being intended for the largest tires. These cables are formed, in known manner, of a core of L wire(s) surrounded by at least one layer of M wires which may itself be surrounded by an outer layer of N wires, with generally L varying from 1 to 4, M varying from 3 to 12, N varying form 8 to 20; if applicable, the assembly may possibly be wrapped by an external wrapping wire wound in a helix around the last layer.
Such layered cables which can be used for reinforcing tire carcasses, in particular carcasses of truck tires, have been described in a very large number of publications. Reference will be made in particular to the documents U.S. Pat. Nos. 3,922,841; 4,158,946; 4,488,587; EP-A-0 168 858; EP-A-0 176 139 or U.S. Pat. No. 4,651,513; EP-A-0 194 011; EP-A-0 260 556 or U.S. Pat. No. 4,756,151; EP-A-0 362 570; EP-A-0 497 612 or U.S. Pat. No. 5,285,836; EP-A-0 568 271; EP-A-0 648 891; EP-A-0 669 421 or U.S. Pat. No. 5,595,057; EP-A-0 709 236 or U.S. Pat. No. 5,836,145; EP-A-0 719 889 or U.S. Pat. No. 5,697,204; EP-A-0 744 490 or U.S. Pat. No. 5,806,296; EP-A-0 779 390 or U.S. Pat. No. 5,802,829; EP-A-0 834 613; WO98/41682; RD (Research Disclosure) No. 34054, August 1992, pp. 624-33; RD No. 34370, November 1992, pp. 857-59.
In order to fulfil their function as reinforcement for tire carcasses, the layered cables must first of all have good flexibility and high endurance under flexion, which implies in particular that their wires are of relatively low diameter, normally less than 0.28 mm, preferably less than 0.25 mm, in particular less than that of the wires used in conventional cables for crown reinforcements of tires.
These layered cables are furthermore subjected to major stresses during running of the tires, in particular to repeated flexure or variations in curvature, which cause friction at the level of the wires, in particular as a result of the contact between adjacent layers, and therefore of wear, and also of fatigue; they must therefore have high resistance to so-called xe2x80x9cfatigue-frettingxe2x80x9d phenomena.
Finally, it is important for them to be impregnated as much as possible with rubber, and for this material to penetrate into all the spaces between the wires forming the cables, because if this penetration is insufficient, there then form empty channels along the cables, and the corrosive agents, for example water, which are likely to penetrate as far as into the tires for example as a result of cuts, move along these channels and into the carcass of the tire. The presence of this moisture plays an important part in causing corrosion and in accelerating the above degradation processes (so-called xe2x80x9cfatigue-corrosionxe2x80x9d phenomena), compared with use in a dry atmosphere.
All these fatigue phenomena which are generally grouped together under the generic term xe2x80x9cfatigue-fretting-corrosionxe2x80x9d are at the origin of gradual degeneration of the mechanical properties of the cables, and may adversely affect the life thereof under very severe running conditions.
In order to improve the endurance of layered cables in truck tire carcasses, in which in known manner the repeated flexural stresses may be particularly severe, it has for a long time been proposed to modify the design thereof in order to increase, in particular, their ability to be penetrated by rubber, and thus to limit the risks due to corrosion and to fatigue-corrosion.
There have for example been proposed or described layered cables of the construction (3+9) or (3+9+15) which are formed of a core of 3 wires surrounded by a first layer of 9 wires and if applicable a second layer of 15 wires, as described, for example, in EP-A-0 168 858, EP-A-0 176 139, EP-A-0 497 612, EP-A-0 669 421, EP-A-0 709 236, EP-A-0 744 490 and EP-A-0 779 390, the diameter of the wires of the core being or not being greater than that of the wires of the other layers. These cables cannot be penetrated as far as the core owing to the presence of a channel or capillary at the centre of the three core wires, which remains empty after impregnation by the rubber, and therefore favorable to the propagation of corrosive media such as water.
The publication RD No. 34370 describes cables of the structure [1+6+12], of the compact type or of the type having concentric tubular layers, formed of a core formed of a single wire, surrounded by an intermediate layer of 6 wires which itself is surrounded by an outer layer of 12 wires. The ability of rubber to penetrate it can be improved by using diameters of wires which differ from one layer to the other, or even within one and the same layer. Cables of construction [1+6+12], the penetration ability of which is improved owing to appropriate selection of the diameters of the wires, in particular to the use of a core wire of larger diameter, have also been described, for example in EP-A-0 648 891 or WO98/41682.
In order to improve further, relative to these conventional cables, the penetration of the rubber into the cable, there have been proposed multi-layer cables having a central core surrounded by at least two concentric layers, for example cables of the formula [1+6+N], in particular [1+6+11], the outer layer of which is unsaturated (incomplete), thus ensuring better penetration ability by the rubber (see, for example, EP-A-0 719 889, WO98/41682). The proposed constructions make it possible to dispense with the wrapping wire, owing to better penetration of the rubber through the outer layer and the self-wrapping which results; however, experience shows that these cables are not penetrated right to the centre by the rubber, and in any case not yet optimally.
Furthermore, it should be noted that an improvement in the ability of the rubber to penetrate is not sufficient to ensure a sufficient level of performance. When they are used for reinforcing tire carcasses, the cables must not only resist corrosion, but also must fulfil a large number of sometimes contradictory criteria, in particular of tenacity, resistance to fretting, high degree of adhesion to rubber, uniformity, flexibility, endurance under repeated flexing or traction, stability under severe flexing, etc.
Thus, for all the reasons set forth previously, and despite the various recent improvements which have been made here or there on such and such a given criterion, the best cables used today in carcass reinforcements for truck tires remain limited to a small number of layered cables of highly conventional structure, of the compact type or the type having cylindrical layers, with a saturated (complete) outer layer; these are essentially cables of constructions [3+9], [3+9+15] or [1+6+12] as described previously.
Now, the Applicant during its research discovered a novel layered cable of the type having an unsaturated outer layer, which unexpectedly improves further the overall performance of the best layered cables known for reinforcing truck tire carcasses. This cable of the invention, owing to a specific structure, not only has excellent ability to be penetrated by the rubber, limiting the problems of corrosion, but also has fatigue-fretting endurance properties which are significantly improved compared with the cables of the prior art. The longevity of truck tires and that of their carcass reinforcements is thus very substantially improved thereby.
Consequently, a first subject of the invention is a multi-layer cable having a unsaturated outer layer, usable as a reinforcing element for a tire carcass reinforcement, comprising a core (C0) of diameter d0 surrounded by an intermediate layer (C1) of six or seven wires (M=6 or 7) of diameter d1 wound together in a helix at a pitch p1, this layer C1 itself being surrounded by an outer layer (C2) of N wires of diameter d2 wound together in a helix at a pitch p2, N being less by 1 to 3 than the maximum number Nmax of wires which can be wound in one layer about the layer C1, this cable being characterised in that it has the following characteristics (d0, d1, d2, p1 and p2 in mm):
(i) 0.14 less than d0 less than 0.28;
(ii) 0.12 less than d1 less than 0.25;
(iii) 0.12 less than d2 less than 0.25;
(iv) for M=6: 1.10 less than (d0/d1) less than 1.40; for M=7: 1.40 less than (d0/d1) less than 1.70;
(v) 5xcfx80(d0+d1) less than p1 less than p2 less than 5xcfx80(d0+2d1+d2);
(vi) the wires of layers C1 and C2 are wound in the same direction of twist.
The invention also relates to the use of a cable according to the invention for reinforcing articles or semi-finished products made of plastics material and/or of rubber, for example plies, tubes, belts, conveyor belts and tires, more particularly tires intended for industrial vehicles which usually use a metal carcass reinforcement.
The cable of the invention is very particularly intended to be used as a reinforcing element of a carcass reinforcement for a tire intended for industrial vehicles selected from among vans, xe2x80x9cheavy vehiclesxe2x80x9dxe2x80x94i.e. subway trains, buses, road transport machinery (lorries, tractors, trailers), off-road vehiclesxe2x80x94agricultural machinery or construction machinery, aircraft, and other transport or handling vehicles.
The invention furthermore relates to these articles or semi-finished products made of plastics material and/or rubber themselves when they are reinforced by a cable according to the invention, in particular tires intended for the industrial vehicles mentioned above, more particularly truck tires, and also to composite fabrics comprising a matrix having a composition of rubber composition reinforced with a cable according to the invention, usable as a carcass reinforcement ply for such truck tires.